The invention relates to a SMD-resistor which comprises a ceramic substrate having two main faces, two side faces and two end faces, and which further comprises two contact layers which are applied to two ends of a main face which adjoin the end faces, a resistive layer which is applied to this main face and electrically contacts both contact layers, as well as two end contacts which cover the end faces of the substrate and which electrically contact the contact layers. The invention also relates to a method of manufacturing SMD-resistors.
The abbreviation SMD stands for "surface mountable device". Unlike conventional resistors, SMD-resistors (also termed chip resistors) have no leads. The end contacts of SMD-resistors can be used to solder them to a so-called PCB (printed circuit board) in a relatively simple manner. By virtue of the absence of leads and the small dimensions of SMD-resistors, a high packing density of said resistors on the PCB can be achieved.
SMD-resistors corresponding to the above description are known per se from, for example, DE-PS 31.04.419. The SMD-resistor described therein comprises a ceramic substrate of alumina. Such a substrate consists of a main phase of sintered Al.sub.2 O.sub.3 -grains which are largely surrounded by a glass-like second phase which keeps the grains together. Contact layers of silver or silver/palladium and a resistive layer are provided on said substrate by means of screen printing. These layers may alternatively be provided by means of other metallizing processes such as sputtering or vapour deposition. The end contacts of the known SMD-resistor comprise a silver or silver/palladium layer which is provided in an immersion process. This layer is provided with a solder layer in an electroplating process. The end contacts may, however, alternatively be provided on the end faces of the substrate by means of an electroless process. In said process, aqueous solutions of Ni and Ag salts in combination with reducing agents are used to provide a thin Ni-layer on the end faces.
The known SMD-resistors have disadvantages. It has for example been found that, in particular, the bonding strength of the end contacts on the end faces of the ceramic substrate is insufficient. This disadvantage occurs in particular when the SMD-resistors are mounted on a PCB. When such a PCB is exposed to mechanical loads such as bending and/or vibrations, fracture may occur between the end contacts and the end faces of the substrate. This may bring about electric interruptions in the conductor pattern of the PCB.